Some refrigerants
have been identified as contributing to ozone depletion and global warming
During the last decade
some refrigerants have been identified as ozone depleting gases and/or
greenhouse gases. As a consequence the chemical companies producing
refrigerants have been working to find alternative refrigerants which have a
good blend of physical and thermodynamic characteristics but do not damage the
environment if they escape. At the same time governments have brought in
legislation which bans the production and use of the more damaging refrigerants.
The most well-known of these pieces of legislation is the Montreal Protocol
on substances which deplete the ozone layer. This legislation
has banned the production of the most ozone depleting refrigerants and has set
a time limit on the manufacture of less damaging refrigerants. Many governments
and the EU have brought in more strict legislation shortening timescales
meaning that bans are now in place.
Refrigerants are
identified in the building services industry by a refrigerant number. For
example, R11 and R12 are chlorofluorocarbons (CFC's) which are highly destructive
to the ozone layer and their production is now banned. R22 is a
hydrochlorofluorocarbon (HCFC) which is less damaging to the ozone layer than
CFC's and so its production is allowed until 2005. Existing stockpiles of both
refrigerants can still be used. R134a is a Hydrofluorocarbon (HFC) it contains no
chlorine and so does not damage the ozone layer. However like other refrigerants
it is a global warming gas.
There are three indices
that are used for comparing the environmental effects of refrigerants:
Ozone Destruction
Potential (ODP) - A measure of how destructive the chemical
is to the ozone layer in comparison to R11 which is said to have an ODP = 1
Atmospheric Lifetime
- The length of time, measured in years,
that the refrigerant remains in the atmosphere causing ozone destruction.
Global Warming
Potential (GWP) - A measure of the contribution the chemical
makes to global warming in comparison to CO2 whose GWP = 1.0.
The table below compares
these indices for various refrigerants. It can be seen that R134a has a zero
ODP but still has a global warming potential. Environmental groups are now campaigning
against HFCs because of their GWP. However, the dominant factor in global warming
is CO2 emitted
(from power stations) as a result of electrical consumption by the chiller
rather than the global warming effect of escaped refrigerants. A method of
quantifying the contributions from each is given by the Total Equivalent
Warming Impact (TEWI). This is a lifecycle analysis which considers both the
direct global warming impact of the escaped refrigerant and the efficiency of
the refrigeration system as a whole.
Refrigerant
|
Type
|
ODP
|
Lifetime
|
GWP
|
R11
|
CFC
|
1.0
|
60
years
|
1500
|
R22
|
HCFC
|
0.05
|
15
years
|
510
|
R134a
|
HFC
|
0.0
|
16
years
|
420
|
R290
|
Propane
|
0.0
|
<1year
|
3
|
R717
|
Ammonia
|
0.0
|
<1year
|
0
|
Lithium
Bromide
|
0.0
|
<1year
|
0
|
|
*Environmental Indices Table
Environmental
concerns have also led to renewed interest in traditional refrigerants such as
ammonia and propane. Both of these do not affect the ozone layer or add to
global warming. There are concerns over toxicity and flammability of these refrigerants
and so they should be used externally and according to appropriate guidelines. Absorption
chillers which use a mix of ammonia/water and waste heat which would otherwise
be wasted have a low contribution on global warming and ozone depletion when
compared to other systems.
